Since the discovery of conducting polymers in 1977, Shirakawa, H.; Lewis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. 1977, 578, extensive research has focused on developing these materials for organic electronic devices. Organic semiconductors such as poly(phenylene vinylene), poly(thiophene), poly(acetylene) and poly(pyrrole) have found application in devices ranging from organic light-emitting diodes (OLEDs) to field effect transistors (FETs). Katz, H. E.; Bao, Z. Journal of Physical Chemistry B 2000, 104, 671-678; Greiner, A. Polymers For Advanced Technologies 1998, 9, 371-389; Gurunathan, K.; Vadivel Murugan, A.; Marimuthu, R.; Mulik, U. P.; Amalanerkar, D. P. Journal of Materials Chemistry and Physics 1999, 61, 173-191. Organic electronics do not match the performance of inorganic, silicon-based devices, due to fundamental limitations on charge carrier mobility in conducting polymers. Katz, H. E.; Bao, Z. Journal of Physical Chemistry B 2000, 104, 671-678. The oft stated advantages of organic electronics, i.e., low cost, mechanical flexibility and ease of processing, could open entire new areas of technical development such as large area displays and disposable electronicsxe2x80x94areas not currently accessible with silicon based devices. However, the most promising organic electronics to date are fabricated using vapor sublimated single crystal films of organic molecules patterned using conventional, silicon-based technology. Katz, H. E.; Bao, Z. Journal of Physical Chemistry B 2000, 104, 671-678; Cui, J.; Huang, Q. L.; Wang, Q. W.; Marks, T. J. Langmuir 2000, 17, 2051-2054. Achieving the goals of low cost and ease of processing requires the development of room temperature, aqueous, solution-based processes for fabricating thin films of conducting polymers.
Both chemical and electrochemical approaches to form thin films of conducting polymers have been developed. Bradley, D. D. C.; Grell, M.; Grice, A.; Tajbakhsh, A. R.; O""Brien, D. F.; Bleyer, A. Optical Materials 1998, 9, 1-11. Sato, M. A.; Sakamoto, M. A.; Miwa, M.; Hiroi, M. Polymer 2000, 41, 5681-5687. Pei, Q. B.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Polymer 1994, 35, 1347-1351. Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. Journal of Electroanalytical Chemistry 1994, 369, 87-92. A variety of monomers can be electropolymerized in polar organic solvents, and these films have been used to make both OLEDs and organic FETs. Tasch, S.; Gao, J.; Wenzl, F. P.; Holzer, L.; Leising, G.; Heeger, A. J.; Scherf, U.; Mullen, K. Electrochemical and Solid State Letters 1999, 2, 303-305; Johansson, T.; Mammo, W.; Andersson, M. R.; Inganas, O. Chemistry of Materials 1999, 11, 3133-3139; Pei, J.; Yu, W. L.; Huang, W.; Heeger, A. J. Macromolecules 2000, 33, 2462-2471; Sainova, D.; Miteva, T.; Nothofer, H. G.; Scherf, U.; Glowacki, I.; Ulanski, J.; Fujikawa, H.; Neher, D. Applied Physics Letters 2000, 76, 1810-1812; Osaka, T.; Komaba, S.; Fujihana, K.; Okamoto, N.; Momma, T.; Kaneko, N. Journal of the Electrochemical Society 1997, 144, 742-748. The main limitation is that polymeric films are typically amorphous and contain a large number of defects. One way to reduce defects is through use of substituted monomers such as 3,4-ethyldioxythiophene (EDOT) or by polymerizing short oligomers such as bithiophene. Kabasakaloglu, M.; Kiyak, T.; Toprak, H.; Aksu, M. L. Applied Surface Science 1999, 152, 115-125.
Even without defects, the amorphous nature of electropolymerized films limits performance in organic devices. In an amorphous conducting polymer film, inter-chain charge hopping leads to non-radiative quenching of electron-hole pairs (excitons). Nanoscale control of the arrangement and orientation of organic molecules can improve the luminescence efficiency of OLEDs and increase the speed of organic FETs. Poly(thiophene) is a hole conductor (p-type) and is electroluminescent, meaning it can be used as either the hole transport or emissive layer of an OLED. Charge injection from an electrode into the hole-transporting layer is improved by orienting polymer chains normal to the electrode surface (parallel to the applied field), compared with a randomly oriented amorphous film. Markart, P.; Zojer, E.; Tasch, S.; Smith, R.; Gin, D.; Leising, G. Synthetic Metals 1999, 102, 1155-1156. Isolating molecules reduces exciton quenching and improves the efficiency of OLEDs. Osterbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. Science 2000, 287, 839-842. For organic FETs, aligning polymer chains is critical for improving device performance, enhancing both carrier mobility and conductivity. Bao, Z. N.; Rogers, J. A.; Katz, H. E. Journal of Materials Chemistry 1999, 9, 1895-1904; Bjornholm, T.; Hassenkam, T.; Greve, D. R.; McCullough, R. D.; Jayaraman, M.; Savoy, S. M.; Jones, C. E.; McDevitt, J. T. Advanced Materials 1999, 11, 1218-1221. This is one reason why single crystal films are often used. Chain alignment within a non-crystalline film enhances carrier mobility while remaining simpler to process.
Poor solubility of conducting polymers is the primary obstacle to using most conventional methods for aligning polymer chains. Thiophene, phenylene vinylene, and other precursors to conjugated polymers are soluble in many organic solvents, especially polar solvents like acetonitrile and tetrahydrofuran (THF). However, the conducting polymers they form are generally insoluble due to their tendency to xcfx80-xcfx80 stack, causing long oligomers or polymers to aggregate in solution. Chemical or electrochemical polymerization of thiophene leads to an intractable, insoluble material that is difficult to characterize or process. Addition of long alkyl chains or other solubizing groups to the 3 or 4 position on the thiophene ring improves solubility. Sato, M. A.; Sakamoto, M. A.; Miwa, M.; Hiroi, M. Polymer 2000, 41, 5681-5687; Kilbinger, A. F. M.; Feast, W. J. Journal of Materials Chemistry 2000, 10, 1777-1784; Tour, J. M.; Wu, R. L. Macromolecules 1992, 25, 1901-1907. Certain substituents even lead to water-soluble monomers and polymers. Stephan, O.; Schottland, P.; Le Gall, P. Y.; Chevrot, C.; Mariet, C.; Carrier, M. Journal of Electroanalytical Chemistry 1998, 443, 217-226. However, large substituents sterically constrain the monomer, inhibiting electropolymerization and degrading electronic properties of the material. Without large solubilizing groups, orientation and alignment of organic semiconductors after polymerization is extremely difficult.
An alternate approach involves the use of a template to position the monomers and lock-in the orientation for alignment during polymerization. Liquid crystals (LCs) are an example of a self-organizing system. One type of lyotropic LC is formed by amphiphilic molecules containing hydrophobic and hydrophilic segments that segregate in a solvent. At low concentrations, amphiphilic molecules form spherical micelles in solution. At higher concentrations, several LC mesophases are possible, including hexagonal, cubic and lamellar structures. The particular mesophase formed depends on a balance between the attractive and repulsive forces on the hydrophilic head group and hydrophobic tail, and the relative volumes of these head/tail segments. Israelachvili, J. Intermolecular and Surface Forces; 2 ed.; Academic Press: San Diego, Calif., 1992; Kunieda, H.; Umizu, G.; Yamaguchi, Y. Journal of Colloid and Interface Science 1999, 218, 88-96.
In light of the foregoing, it is an object of the present invention to provide general methodologies relating to the use of liquid crystals to template the electropolymerization of precursors for conducting or light-emissive compositions. Various related objectives of this invention can be illustrated by comparison with the prior art.
For instance, P. Braun, et. al. used a liquid crystal to directly template hexagonal superlattices of cadmium sulfide. Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325-328; Braun, P. V.; Osenar, P.; Tohver, V.; Kennedy, S. B.; Stupp, S. I. Journal of the American Chemical Society 1999, 121, 7302-7309. In this case water soluble cadmium precursors segregated to the hydrophilic regions of the hexagonal LC mesophase. Accordingly, it is an object of the present invention to utilize relatively non-polar, organic precursors, which should segregate to and be confined within the hydrophobic core of an LC mesophase. As a related object, electropolymerization of monomers within the hydrophobic cores should result in polymers aligned by the LC mesophase and electronically isolated within the hydrophobic cores.
D. Gin et. al. reported a method for a water-soluble, thermally convertible poly(phenylene vinylene) (PPV) based precursors confined within aqueous channels of an inverse hexagonal mesophase. Markart, P.; Zojer, E.; Tasch, S.; Smith, R.; Gin, D.; Leising, G. Synthetic Metals 1999, 102, 1155-1156; Gin, D.; Smith, R.; Deng, H.; Leising, G. Synthetic Metals 1999, 101, 52-55. However, the inverse hexagonal (H2) mesophase incorporated a photopolymerizable group. After UV exposure to cross-link the LC mesophase, the material is heated to form a substituted poly(phenylene vinylene) (PPV) in situ within the hydrophilic channels of the structure. While this approach of the prior art is limited to water soluble, thermally polymerizable precursors, an object of the present invention is to extend associated methodologies to a wider range of conducting polymers.
S. Tolbert, et. al clearly demonstrated the ability to control energy transfer and enhance electro-optical properties of semiconducting polymers by isolating them within nanoscale channels. Wu, J. J.; Gross, A. F.; Tolbert, S. H. Journal of Physical Chemistry B 1999, 103, 2374-2384; Nguyen, T. Q.; Wu, J. J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652-656; Nguyen, T. Q.; Wu, J.; Tolbert, S. H.; Schwartz, B. J. Advanced Materials 2001, 13, 609-+. A surfactant, water and a silica precursor were used to template nanoporous silica with hexagonal symmetry. Substituted PPV molecules are inserted into the pores by soaking samples in an organic solution. Though quite useful, templating via mesoporous silica is limited to soluble polymers, since the material is not polymerized in situ. This process of the prior art also requires calcination at high temperature (500xc2x0 C.) to form the silica template. As such, an object of this invention is to provide a templating methodology accomplished at or near room temperature and to extend the templating methodology to precursors that form insoluble polymers.
Bayer AG developed the first commercialized conducting polymer, using poly(styrene sulfonate) (PSS) as a polyelectrolyte to solubilize 3,4-ethyldioxythiophene (EDOT) monomer in water. Groenendaal, B. L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Advanced Materials 2000, 12, 481-494. Polymerization results in a blend of poly(3,4-ethyldioxythiophene) (PEDOT) and PSS. While this method of the prior art solubilizes the otherwise insoluble PEDOT, it does not result in any ordering of the polymer. Films cast from the PEDOT-PSS solution are amorphous because the PSS forms a micellar solution. Accordingly, it is an object of the present invention to provide a templating strategy that will order, orient and/or advantageously align conducting polymers.
C. Henry, et. al., Henry, C.; Armand, F.; Araspin, O.; Bourgoin, J. P.; Wegner, G. Chemistry of Materials 1999, 11, 1024-1029, described electropolymerization of substituted thiophene precursors in an oriented alkylcellulose. However, domain size within the cellulose structure was on the order of microns, and thus could not result in confinement of individual molecules. In addition, such an alkylcellulose structure, though having a preferred orientation, is not ordered in multiple dimensions. Accordingly, other objects of the present invention include use of LCs ordered in multiple dimensions and/or on a nanoscale to better utilize the benefits available through a template strategy.
Other objects, features, benefits and advantages of the present invention will be apparent from the following summary and descriptions of various preferred embodiments, and will be readily apparent to those skilled in the art having knowledge of various templating, electropolymerization and/or device fabrication techniques. Such objects, features, benefits and advantages will be apparent from the above as taken into conjunction with the following examples, data, figures and all reasonable inferences to be drawn therefrom.